Tracking visual search over space and time

Visual perception consists of early preattentive processing and subsequent attention-demanding processing. Most researchers implicitly treat preattent ive processing as a domain-dependent, indivisible stage. We show, however, by interrupting preattentive visual processing of color before its completi on, that it can be dissected both temporally and spatially. The experiment depends on changing easy (preattentive) selection into difficult (attention -demanding) selection. We show that although the mechanism subserving preat tentive selection completes processing as early as 200 msec after stimulus onset, partial selection information is available well before completion. F urthermore, partial selection occurs first at locations near fixation, spre ading radially outward as processing proceeds.